Satellite images indicate that China has deployed an operational airship known as an aerostat in the disputed Mischief Reef , strengthening its reconnaissance ability in the South China Sea . An airship-shaped object could be seen floating over the atoll, which is now China’s biggest outpost in the disputed waters, Israeli satellite company ImageSat International (ISI), said in a post on its Twitter account on Sunday. The photos, taken by ISI on November 18, was the first evidence that China had deployed such an aircraft in the area, the company said. China reportedly began building an early-warning system of aerostats in 2017. Huge balloons were fitted with phased-array radars to help detect low-flying incoming planes, according to military magazine Kanwa Asian Defence . The craft can remain aloft for an extended period of time, offering a relatively low-cost, efficient and all-weather solution to monitor a large area when spy planes cannot be deployed. When teamed with ground radar, satellites and early-warning reconnaissance planes, they can form a comprehensive surveillance network. Aerostats are being deployed in some of China’s strategic hotspots such as the country’s border with North Korea and the Taiwan Strait. According to Kanwa Asian Defence , the high-powered aerostats can monitor both airborne targets and mobile ground objects within a radius of 300km (186 miles). The Chinese military has significantly expanded its reach in the South China Sea, setting up radars, deploying missiles and missile launchers and warplanes on islands and reefs in the region, where Vietnam, the Philippines, Malaysia, Brunei and Taiwan also have overlapping claims. Vietnam and China promise to keep talking as they look to settle differences over South China Sea Mischief Reef is located on the eastern edge of seven artificial islands built by China in the Spratly archipelago. The reef covers 5.5 sq km (2.1 square miles) and boasts an airfield capable of handling an Airbus A320. It also has a lighthouse and other military and civilian facilities. In 2016, an international tribunal ruled that because the outpost was a reef that was submerged at high tide, it did not qualify for 12-nautical mile territorial waters. It has since become a focus of the US Navy’s challenges to China’s “excessive claims” in the waters, and has been the target in six of 21 “freedom of navigation” operations in the past few years. Just 10 days ago, the USS Gabrielle Giffords sailed within 12 nautical miles of Mischief in one of those operations. The Chinese defence ministry said the Chinese side “tracked, monitored, identified, warned and expelled” the Independence-class littoral combat ship.